With issues of fossil fuel depletion or global warming as a backdrop, a great attention has recently been given to solar cells as a clean energy source. Research and development on them are being actively conducted. Silicon-based solar cells, typified by monocrystalline Si, polycrystalline Si, or amorphous Si solar cells, have been in practical use, but they are expensive, and shortage of the material Si has surfaced. In such a situation, demand for next-generation solar cells has been growing.
In response to the demand, organic solar cells such as dye-sensitized solar cells, organic thin film solar cells, and organic-inorganic hybrid solar cells have received attention in recent years.
A dye-sensitized solar cell typically includes as an electrode material a layer of titanium oxide formed on a base that includes a conductive layer (electrode). This titanium oxide layer serves to 1) adsorb sensitizing dye, 2) accept electrons injected from excited sensitizing dye, 3) transport electrons to the conductive layer, 4) offer a reaction site for electron transfer (reduction) from an iodide ion to dye, and 5) scatter and confine light. This layer is one of the most important factors to determine performances of the solar cell. When the titanium oxide layer serves to “1) adsorb sensitizing dye,” the titanium oxide layer is required to absorb much sensitizing dye to improve photoelectric conversion efficiency. Accordingly, the titanium oxide layer is required to be porous, to have as large a surface area as possible, and to contain as little impurities as possible.
The titanium oxide layer as an electrode material is also used in organic thin film solar cells and organic-inorganic hybrid solar cells. In organic thin film solar cells and organic-inorganic hybrid solar cells, the titanium oxide layer is overlaid with a semiconductor, in which electrons and holes are generated by light excitation. To improve photoelectric conversion efficiency, the titanium oxide layer is required to have a larger contact area with the semiconductor, and therefore is required to be porous as in dye-sensitized solar cells.
Such a porous titanium oxide layer is generally formed by applying a paste containing titanium oxide particles and an organic binder on a base, volatilizing the solvent, and then removing the organic binder by high-temperature firing. This produces a porous layer in which the titanium oxide particles are sintered together and which has many fine voids in the layer. However, high-temperature firing at temperatures higher than 500° C. rules out the use of resin bases, which are increasingly needed these days for further reduction in costs. Low-temperature firing disadvantageously allows residues of the organic binder to be left on the surface of the titanium oxide particles, thus significantly reducing photoelectric conversion efficiency.
To overcome these problems, Patent Literature 1, for example, discloses a firing treatment at a low temperature using a paste which contains a reduced amount of an organic binder. The paste of Patent Literature 1, however, has a low viscosity, and thus makes it difficult to retain the shape of the applied paste. This paste therefore causes non-uniform film thickness and loss of shape of the film end, and when printed in a pattern of micro wiring, it causes coalescence of the wiring.